1. Field of the Invention
The present invention relates to a spark-ignited direct cylinder fuel injection engine which has spark plugs and fuel injection valves spraying a fuel mist directly into each cylinder, and more specifically to a spark-ignited direct cylinder fuel injection engine in which an air-fuel mixture having an ignitable air-fuel ratio is formed in a region near a spark plug in a combustion chamber by direct fuel injection, this mixture being ignited by means of the spark plug and enabling operation with an overall lean gas mixture.
2. Description of the Related Art
An example of this type of spark-ignited direct cylinder fuel injection engine is disclosed, for example, in the Japanese Unexamined Patent Publication (Kokai) No. 5-231155. In the engine disclosed by JPP'155, a combustion chamber is formed in a shape of a round depression in the top surface of a piston, and a depressed spark plug pocket is formed in wall of the combustion chamber. Fuel injected from a direct fuel injection valve into the combustion chamber forms a gas mixture having a combustible air-fuel ratio inside a plug pocket, and is ignited by a spark from a spark plug.
Additionally, in the engine of JPP'155, the fuel injection valve is such that it generates a fuel jet having a high penetrating force with a narrow spray angle, and injects the fuel so that it strikes the combustion chamber wall surface upstream from the spark plug pocket. As a result, part of the injected fuel is atomized by the collision with surface of the wall of the combustion chamber, this atomized fuel flowing toward the spark plug pocket along the surface of the wall of the combustion chamber, and then flowing into the spark plug pocket. When this occurs, part of the fuel receives heat from the high-temperature surface of the wall of the combustion chamber, thereby being vaporized and resulting in the formation of a air-fuel mixture inside the spark plug pocket.
As described above, even if the amount of injected fuel is small in relation to the amount of air which is taken into the combustion chamber, since the fuel is collected at the spark plug pocket, its concentration is higher inside the spark plug pocket than in other parts of the combustion chamber, whereby an air-fuel mixture having a combustible air-fuel ratio is formed. Therefore, the air-fuel mixture in the spark plug pocket can be ignited by means of the spark plug even though the air-fuel ratio of the mixture outside of the spark plug pocket is too lean to be ignited.
The spark-ignited direct cylinder fuel injection engine described in JPP'155 offers the advantage that, during low-load running, stable operation of the engine becomes possible even when the amount of injected fuel is small, because the fuel is collected in proximity to the spark plug, thus forming an air-fuel mixture having an air-fuel ratio of the ignitable range. However, in an engine such as the one disclosed in JPP'155, if the shape of the combustion chamber is established so that the injected fuel is gathered near the spark plug to form an air-fuel mixture having higher concentration of fuel, there are cases in which problems such as a worsening of ignition, smoke generation, and spark plug fouling occur when the engine is operated with a high load.
When the engine is operated at a high load, the amount of fuel injected into the combustion chamber is increased. Therefore, if the shape of the combustion chamber is established such that injected fuel is collected in the spark plug pocket, a large amount of fuel flows into the spark plug pocket in a high load operation of the engine. This results in an excessively high concentration within the spark plug pocket, which causes misfiring and incomplete combustion and these, in turn, cause smoke to be generated and carbon deposits to be left on the spark plugs.
In an engine such as disclosed in JPP'155, these problems occur because the spark plug pocket which collects injected fuel is provided in the combustion chamber. However, even with a combustion chamber having no spark plug pocket, in engines performing so called a stratified combustion in which fuel is collected in the region near a spark plug to form an air-fuel mixture of an ignitable air-fuel ratio when running at low load, similar problems occur. Namely, in the engines performing a stratified combustion, the air-fuel mixture in the area surrounding the spark plug has an excessively high concentration when the amount of injected fuel increases at high engine loads. Therefore, similar problems, such as a worsening of ignition, generation of smoke and fouling of spark plugs, may occur.
If the shape of the combustion chamber is established so as to diffuse the fuel uniformly within the combustion chamber, these problems in the high load operation of the engine may be avoided. However, in this case, the air-fuel mixture becomes excessively lean in the low load operation of the engine since a small amount of fuel in the low load operation is diffused uniformly within the combustion chamber. This causes a failure to ignite air-fuel mixture.